Carbon-carbon composite brake discs and carbon-carbon-silicon carbide composite brake discs are subject to oxidation in their normal operating environments. Such composite brake discs have been protected against oxidation by coating them with phosphoric acid-based penetrants. However, currently used phosphoric acid-based penetrants are subject to degradation at peak operating conditions.
Carbon-carbon composites are a class of unique materials whose properties, especially at elevated temperatures, make them attractive for various aerospace applications. The materials are composites, although often all the composite elements are comprised essentially of carbon in its various allotropic forms. Factors such as the degree of graphitization, purity, pore structure, specific surface areas, surface complexes, oxygen availability, and temperature have a strong influence on the oxidation of carbon materials. Oxidation of carbon-carbon composites at elevated temperatures in an atmospheric environment has a catastrophic effect on the mechanical properties of the composites. Some form of oxidation protection is required in order to maintain structural capability.
One approach is the application of a refractory ceramic barrier coating. SiC is known to be a useful coating for this purpose. U.S. Pat. No. 5,283,109 teaches a silicon carbide-coated carbon composite formed with a carbon interlayer. The carbon interlayer is prepared by coating a carbon composite base with a paste-like mixture of carbon powder and a liquid carrier followed by curing. The coated carbon composite is then subjected to chemical vapor deposition with silicon carbide. An open porous layer is needed to allow penetration of the chemical vapor. This final coated composite is inadequate for applications such as airplane brakes, due to a tendency for the composite to crack or peel under extreme conditions as a result of the relatively weak bond between the carbon interlayer and either the carbon composite base or the silicon carbide layer.
Forsythe and Walker disclose (U.S. Pat. No. 6,555,173 B1 and U.S. Pat. No. 6,756,121 B2) a silicon carbide-coated C—C composite that is resistant to oxidation at high temperatures. The composite is prepared by coating a C—C composite base with a reactive carbon-containing composition followed by applying a silicon-containing composition to the reactive carbon-coated C—C composite to form a bi-layered C—C composite, then heating the bi-layered C—C composite to at least the melting point of silicon to form the silicon carbide-coated C—C composite, and optionally applying a phosphorus-containing retardant solution to the silicon carbide-coated C—C composite.